Electric power steering control system

ABSTRACT

An electric power steering control system in which the steering &#34;feel&#34; is improved, having a steering force detector an electric motor for producing an auxiliary steering force, a motor current detector and a control unit for deciding the motor current value and rotational direction at least in accordance with the detected steering force. The motor current is limited in accordance with the detected steering force and motor current values.

This is a divisional of application Ser. Nos. 08/532,014 filed Sep. 21,1995, issued Nov. 24, 1998 and 09/162,515 filed Sep. 29, 1998, andissued Nov. 23, 1999 now U.S. Pat. Nos. 5,839,537 and 5,988,310 thedisclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to an electric power steering control system withimproved motor control performance and steering "feel", withoutaffecting safety.

DESCRIPTION OF THE RELATED ART

FIG. 35 shows a known electric power steering control system for use inan automobile. In this drawing, a torque sensor 15 is disposed on aninput shaft 201 of a steering wheel 200, and connected to a control unit20. A vehicle speed sensor 16 is also connected to the control unit 20,and the current of an electric assist motor 1 is controlled according totorque and vehicle speed. The motor 1 is coupled with an output shaft203 through a clutch 13 and a reduction gear 202. The clutch 13 iscontrolled by the control unit 20. When the clutch 13 is engaged, theoutput shaft 203 and a pinion 204 rotate according to the rotation ofthe motor 1, thereby driving a rack 205 to move either leftward orrightward, and front wheels 206 connected to the rack are steered. Sincethe output shaft 203 is also connected to the input shaft 201 throughthe reduction gear 202, a desirable steering feel is provided. The motorcurrent characteristics are shown in FIG. 7; the electric motor isdriven in a direction to reduce the steering power.

Japanese Kokai No. 1-257674 discloses an arrangement wherein torquesignal direction judgment -means for detecting the steering directionfrom the steering torque are provided to prevent the motor from beingdriven in the reverse direction of the steering torque.

By incorporating this technique in the control unit 20 of FIG. 35, anauxiliary steering force against the steering direction is not producedby the motor 1 even if the computer in the control unit 20 malfunctions,thus improving safety.

In an electric power steering control system of such an arrangement,however, since the motor is connected to the steering system, therearise such disadvantages as an increase in inertial mass or an increasein torque loss, thereby deteriorating the steering feel. To improve thesteering feel, several attempts such as inertia compensation, torqueloss compensation, etc. have been heretofore proposed.

In a control system without inertia compensation, when steering isperformed as shown in FIG. 8(a), the motor is not yet caused to rotateat the time 1 when the steering is started and therefore steering forceis increased by an amount necessary to rotate the motor. Further, at thetime 2 of the start of the holding state, overshoot which takes placedue to inertia of the motor results in deterioration of steering feel.When performing inertia compensation, it is possible to compensate theinertial sensation of the motor by applying a current sufficient tooffset the inertia produced when the rotation of the motor is started,as shown in FIG. 8(e) at the time , or by applying a current sufficientto offset the inertia produced when the rotation of the motor isstopped, as shown in FIG. 8(e) at the time 2, thereby improving thesteering feel.

In the control system without torque loss compensation, when thesteering wheel is returned to neutral without holding the steering wheelafter steering right, as shown in FIG. 13(a), the steering wheel isreturned to neutral by driving the motor due to a reaction from the roadsurface. Therefore disadvantages arise of a reduction in the returningspeed of the steering wheel corresponding to the torque loss of themotor and the steering wheel not returning to neutral. When performingtorque loss compensation, the steering wheel returning performance canbe improved by supplying a current to compensate for the torque loss ofthe motor to the motor at the time the steering wheel is returned, asshown in FIG. 13(e).

As mentioned above, in the conventional inertia compensation and torqueloss compensation system, feel is improved by supplying a motor currentin the reverse direction of the steering torque. A problem, however,exists in that since the electric motor is inhibited from being drivenin the reverse direction of the steering torque, controls such asinertia compensation control and torque loss compensation control arenot available, so that steering feel is not sufficiently improved.

SUMMARY OF THE INVENTION

The present invention solves the above-described problems. It is anobject of the invention to provide an electric power steering controlsystem in which steering feel is improved, and in which motor controlcan be performed without affecting safety.

It is another object of the invention to drive the motor in a reversedirection to the steering torque under the condition of a limited motorcurrent.

It is a further object of the invention to improve steering feel bymeans of inertia compensation control.

It is yet another object of the invention to provide an electric powersteering control system wherein dangerous torque is not produced when acurrent is applied in the reverse direction of the driving torque.

An electric power steering control system according to the inventioncomprises: steering force detecting means for detecting a steeringforce; an electric motor for producing an auxiliary steering force;motor current detecting means for detecting a motor current of saidelectric motor; motor current control means for determining a motorcurrent value and a driving direction of the electric motor required bysaid electric motor at least in accordance with the steering forcedetected by said steering force detecting means, thereby controlling themotor current; and motor current limiting means for limiting said motorcurrent in accordance with the steering force detected by said steeringforce detecting means and the motor current value detected by said motorcurrent detecting means.

An electric power steering control system according to the inventioncomprises: steering force detecting means for detecting a steeringforce; an electric motor for producing an auxiliary steering force; apower source for supplying electric power to said electric motor; motorcurrent control means for determining a motor current of said motorcurrent value and a driving direction of the electric motor required bysaid motor at least in accordance with the steering force detected bysaid steering force detecting means, thereby controlling the motorcurrent in the form of a pulse width modulation control; a bridgecircuit for driving said electric motor; a sample-and-hold circuit forsampling and holding a current between said power source and said bridgecircuit or a current between said bridge circuit and the groundsynchronously with a pulse width modulation signal; motor currentdetecting means for detecting a motor current using said sample-and-holdcircuit; and abnormal motor current detecting means for detecting anabnormality in the motor current in accordance with a motor currentvalue detected by said motor current detecting means; characterized inthat said sample-and-hold circuit is operated so as to perform thedetection of motor current even when said electric motor is not operatedaccording to said motor control means.

An electric power steering control system according to the inventioncomprises: steering force detecting means for detecting a steeringforce; an electric motor for producing an auxiliary steering force;motor current detecting means for detecting a motor current of saidelectric motor; motor current control means for determining a motorcurrent target value and a driving direction of the electric motorrequired by said electric motor at least in accordance with the steeringforce detected by said steering force detecting means, therebycontrolling the motor current; motor current limiting means forcomparing said motor current target value with a predetermined value soas to limit said motor current within a predetermined range; andabnormal motor current detecting means for detecting an abnormality inthe motor current in accordance with said motor current target value andsaid detected motor current value; wherein said motor is controlled bysaid motor control means within a range not subject to limitation bysaid motor current limiting means, wherein current error does not takesplace between the indicated value of motor current and the detectedvalue of motor current as a result of the establishment of a targetvalue of motor current within a range limited by the motor currentlimiting means.

In addition, in an electric power steering control system comprisingmotor current detecting means for detecting a motor current using asample-and-hold circuit, abnormal motor current detecting means fordetecting abnormality in the motor current in accordance with thedetected motor current value, and motor current limiting means forlimiting the motor current in accordance with the steering torque anddetected motor current value, since the detection of motor current atthe time when the electric motor is not driven becomes possible byturning on the sample-and-hold circuit when the electric motor is notdriven, detection of abnormality in the motor current is possible.Furthermore, it is possible to limit the motor current in the reversedirection of the steering torque by the motor current limiting means,resulting in a safer electric power steering control system.

Other objects and advantages of this invention will become apparent fromthe detailed description given hereinafter. It should be understood,however, that the detailed description and specific embodiment areprovided by way of illustration only, and that various changes andmodifications within the spirit and scope of the invention will becomeapparent to the those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic circuit diagram of the electric power steeringcontrol system common to embodiments 1, 2, 5, 9 of the invention.

FIG. 2 is a detailed circuit diagram of the motor current limiting meansaccording to embodiment 1 of the invention shown in FIG. 1.

FIG. 3 is a graph showing the characteristic of the torque sensor 15according to an embodiment of the invention shown in FIG. 1.

FIG. 4 is a graph showing the characteristic of the outputs ILL and ILRof the motor current limiting means according to the embodiment of theinvention shown in FIG. 2.

FIGS. 5(a) to 5(i) are a timing chart to explain the operation of themotor current detecting means according to an embodiment of theinvention shown in FIG. 1.

FIG. 6 is a control block diagram showing the control of the electricpower steering control system according to an embodiment of theinvention shown in FIG. 1.

FIG. 7 is a graph showing the motor current characteristic to explainthe output of the target current deciding means according to anembodiment of the invention.

FIG. 8 is a waveform diagram to explain the electric power steeringoperation provided with inertia compensation control according to anembodiment of the invention shown in FIG. 1.

FIGS. 9(a) to 9(i) are a timing chart to explain the operation of anembodiment of the invention shown in FIG. 1 when the motor currentlimiting means is abnormal.

FIGS. 10(a) and 10(b) are graphs showing the characteristics of theoutputs ILR and ILL of the motor current limiting means other than thoseshown in FIG. 4 according to an embodiment of the invention.

FIGS. 11(a) and 11(b) are graphs showing the characteristics of theoutputs ILR and ILL of the motor current limiting means other than thoseshown in FIG. 4 according to an embodiment of the invention.

FIG. 12 is a control block diagram of the electric power steering havingtorque loss compensation according to embodiment 2 of the invention.

FIGS. 13(a) to 13(f) are a timing chart to explain the operation of theelectric power steering provided with torque loss compensation accordingto embodiment 2 of the invention shown in FIG. 12.

FIG. 14 is a schematic circuit diagram showing the control unit of theelectric power steering control system according to embodiment 3 of theinvention.

FIG. 15 is a flow chart to explain the control according to embodiment 3of the invention shown in FIG. 14.

FIG. 16 is a flow chart showing the abnormal motor current detectionprocessing according to embodiment 3 of the invention shown in FIG. 14.

FIG. 17 is a schematic circuit diagram of embodiment 4 of the invention.

FIG. 18 is a flow chart showing the control according to the embodiment5 of the invention.

FIG. 19 is a circuit diagram showing one embodiment of the motor currentlimiting means employed in embodiment 5 of the invention.

FIG. 20 is a flow chart showing the control when employing the motorcurrent limiting means in FIG. 19.

FIG. 21 is a schematic circuit diagram of embodiment 6 of the invention.

FIG. 22 is a flow chart showing the control according to embodiments 6and 7 of the invention.

FIG. 23 is a flow chart showing the abnormal motor current detectionprocessing according to embodiment 6 of the invention.

FIG. 24 is a schematic circuit diagram of embodiment 7 of the invention.

FIG. 25 is a flow chart showing the abnormal motor current detectionprocessing according to the embodiment 7 of the invention.

FIG. 26 is a schematic circuit diagram of embodiment 8 of the invention.

FIG. 27 is a circuit diagram showing the motor current limiting meansaccording to embodiment 8 of the invention.

FIGS. 28(a) and 28(b) are graphs showing the operation of the circuit inFIG. 27.

FIGS. 29(a) to 29(j) are a timing chart showing the operation ofembodiment 8 of the invention.

FIGS. 30(a) to 30(j) are a timing chart showing the operation of theembodiment 8 of the invention.

FIGS. 31(a) to 31(j) are a timing chart showing the operation ofembodiment 8 of the invention.

FIG. 32 is a circuit diagram of the motor current limiting meansaccording to embodiment 9 of the invention.

FIG. 33 is a graph showing the operation of the motor current limitingmeans of FIG. 32.

FIGS. 34(a) to 34(e) are a timing chart showing the operation of theembodiment 9 of the invention.

FIG. 35 is a block diagram showing an arrangement of the prior electricpower steering control system.

In all figures, elements which are the same or substantially the samehave the same reference numbers.

DESCRIPTION OF PREFERRED EMBODIMENTS Embodiment 1

An electric power steering control system incorporated in an automobileaccording to embodiment 1 of the present invention is hereinafterdescribed with reference to FIGS. 1 to 11. This embodiment relates to amanner of steering. An electric power steering control system shown inFIG. 1 has an electric motor 1 for producing an auxiliary steeringforce; a battery 2 for supplying power to the electric motor 1; a relay3; transistors 4a-4d constituting a bridge arrangement for driving theelectric motor 1; transistor drive circuits 5a-5d for respectivelydriving the transistors 4a-4d; AND circuits 6a-6d; a shunt resistor 7for detecting a motor current; an analog switch 8 for turning on when apulse width modulation (hereinafter referred to as PWM) signal is highand turning off when the PWM signal is low; a capacitor 9 for holding avoltage when the analog switch 8 is turned on; a resistor 10 fordischarging an electric charge from the capacitor 9 in conformity withthe time constant of the motor 1; a sample-and-hold circuit 11comprising the analog switch 8, capacitor 9 and resistor 10; anamplifier circuit 12 for amplifying signals from the sample-and-holdcircuit 11; a clutch 13 for mechanically connecting or disconnecting themotor 1 and the steering system; a clutch drive circuit 14 for drivingthe clutch 13; a torque sensor 15 for detecting a steering torque; avehicle speed sensor 16 for detecting vehicle speed; a motor rotationspeed sensor 17 for detecting rotation speed of the motor 1; amicrocomputer 18 serving as motor control means for controlling themotor current in accordance with signals from the torque sensor 15,vehicle speed sensor 16 and motor rotation speed sensor 17; motorcurrent limiting means 19 for limiting the motor current in accordancewith the steering torque signal TRQ of the torque sensor and the motorcurrent detection signal IMD; and a control unit 20 for the electricpower steering control system.

The motor current limiting means 19, which are now described withreference to FIG. 2, comprises a 5V power source Vcc, resistors 50, 51,52, 55, 56, comparators 53, 54, 57, and OR circuits 58 and 59, whereinthreshold values TH1 and TH2 are decided and compared with the inputtorque signal TRQ by comparators 53 and 54. A threshold value TH3 isdecided and compared with the input detected motor current value IMD bya comparator 57.

The steering torque signal TRQ of the torque sensor 15 has acharacteristic corresponding to the steering torque as shown in FIG. 3.The output of the comparator 53 is high when the steering torque signalTRQ is higher than the predetermined value TH1 (a right torque of 1N·m,for example), and the output of the comparator 53 is low when thesteering torque TRQ is less than the predetermined value TH1.

On the other hand, the output of comparator 54 is high when the steeringtorque signal TRQ is less than the predetermined value TH2 (a lefttorque of 1N·m, for example), and the output of comparator 54 is lowwhen the steering torque TRQ is greater than the predetermined valueTH2.

Further, the comparator 57 compares the detected motor current value IMDwith a predetermined value TH3, and the output of comparator 57 is highwhen IMD is less than TH3 (under 3 Å, for example), and the output ofcomparator 57 is low when the detected motor current value is greaterthan the predetermined value TH3. As the comparators 53, 54 and 57 areconnected to the OR circuits 58, 59, the outputs ILL, ILR are both highwhen the detected motor current value IMD is under the predeterminedvalue TH3, and the outputs ILL and ILR are high or low as a result ofcomparison with the predetermined values TH1 and TH2 as respectivelyshown in FIG. 4(a) and FIG. 4(b) when the detected motor current valueIMD is over the predetermined value TH3.

Operation of the control unit 20 of the electric power steering controlsystem is hereinafter described. In the rightward drive, a PWM signalfor PWM drive of the motor 1 is output from a PWM port of themicrocomputer 18. When the port R is high, port L is low, output ILR ofthe motor current limiting means 19 is high and ILL is low, thetransistor 4a is in the PWM operation, the transistor 4b is off,transistor 4c is on, and transistor 4d is off, whereby a current passesin the direction A through the motor 1 and a rightward auxiliary forceis produced.

On the other hand, in the leftward drive, when the port R is low, port Lis high, output ILR of the motor current limiting means 19 is low andILL is high, the transistor 4a is off, the transistor 4b is in the PWMoperation, transistor 4c is off, and transistor 4d is on, whereby acurrent flows in the direction B in the motor 1 and a leftward auxiliaryforce is produced.

Further, when the detected motor current value IMD is greater than thepredetermined value TH3, the signals ILR, ILL from the AND circuits 6c,6d and the motor current limiting means 19 are low, whereby thetransistors 4c, 4d cannot drive the motor in the reverse direction ofthe steering torque.

Operation of the sample-and-hold circuit 11 for detecting the motorcurrent is hereinafter described with reference to FIG. 5, taking therightward drive of the electric motor as an example. In this drivingoperation, the port R is high, port L is low, output ILR of the motorcurrent limiting means 19 is high and output ILL is low. In FIG. 5, (a)denotes a PWM signal, (b) denotes an operation waveform of thetransistor 4a, (c) denotes an operation waveform of the transistor 4b,(d) denotes an operation waveform of the transistor 4c, and (e) denotesan operation waveform of the transistor 4d. During the period I when thetransistors 4a, 4c are on and 4b, 4d are off, a current passes from thebattery 2 to the ground through the relay 3, transistor 4a, motor 1,transistor 4c and shunt resistor 7, and during the period II when thetransistors 4a, 4b, 4d are off and 4c is on, a flywheel current passesin the direction A from the motor 1 to the motor 1 through diodesincorporated in the transistors 4c, 4d by inductance of the motor 1, sothat the motor current waveform is as shown in FIG. 5(f).

During the period I a motor current passes through the shunt resistor 7,and during the period II no motor current passes therethrough, and as aresult, a waveform detected by the shunt resistor 7 is as shown in FIG.5(g). The analog switch 8 is turned on/off at the timing shown in FIG.5(h) corresponding to the PWM signal. During the period I when theanalog switch 8 is on, the capacitor 9 is charged with a voltagedetected by the shunt resistor 7, and during the period II when theanalog switch 8 is off, the electric charge charged during the period Iis discharged through the resistor 10 resulting in a waveform as shownin FIG. 5(i). A signal corresponding to the motor current can beobtained by establishing a discharge time constant so that the dischargewaveform 2 at this time may be equal to 1 of the motor current waveform(f).

The control manner of the system shown in FIG. 1 is hereinafterdescribed with reference to FIG. 6. The steering torque TRQ is detectedby the torque sensor 15, the vehicle speed VSP is detected by thevehicle speed sensor 16, and the target value IMT of the motor currentis decided by the target current deciding means 100. Inputcharacteristics of the target current deciding means 100 are shown inFIG. 7, in which current is caused to pass so as to produce a rightwardauxiliary force when producing a rightward torque, while current iscaused to pass so as to produce a leftward auxiliary force whenproducing a leftward torque.

The motor current is also changed depending upon the vehicle speed so asto produce an auxiliary steering force corresponding to the vehiclespeed. Further, a motor rotation speed signal NM is detected from themotor rotation speed sensor 17, a current IMC for the inertiacompensation is decided by the inertia compensation means 101, and anindicated value IMI of the motor current is obtained by adding the motorcurrent target value IMT and the inertia compensation current IMC in anadder 102. In subtractor 103, a deviation ΔIM between the indicatedvalue IMI of the motor current and the detected motor current value IMDdetected by the motor current detecting circuit 105 is obtained, and acontrol is performed by the motor drive means 104 so that ΔIM may bezero.

Operation when the steering wheel is steered rightward is hereinafterdescribed with reference to the timing chart in FIG. 8. In the drawing,(a) denotes a steering angle θ of the steering wheel, (b) denotes acurrent IMC for compensating the inertia component of the electric motor1 by the inertia compensation means, (c) denotes a steering torque TRQ,(d) denotes a target value IMT of the motor current decided by thesteering torque TRQ and the vehicle speed VSP, (e) denotes a motorcurrent indicated value IMI obtained by adding the motor current targetvalue IMT and the inertia compensation current IMC, and (f) denotes arotation speed NM of the electric motor.

In this embodiment, inertia of the electric motor 1 is compensated byproportioning the inertia compensation current IMC to a parameter of themotor rotation speed NM, and the inertia feeling is reduced by applyingrespectively a current for compensating a starting torque of theelectric motor at the moment 1 and a current for compensating a torquefor stopping the electric motor at the moment 2. Though the direction isopposite to that of the steering torque at the moment 2, the current forcompensating the inertia of the motor 1 is under the threshold value TH3(3 Å, for example) and, therefore, the current is not subject tolimitation by the motor current limiting means 19.

A state in which something abnormal takes place in the microcomputer 18and the motor is driven irrespective of the steering torque ishereinafter described with reference to FIG. 9. This drawing shows astate in which the microcomputer 18 malfunctions so as to drive themotor rightwardly at the moment 1 when the steering torque is neutral,and in which (a) denotes operation of the transistor 4a, (b) denotesoperation of the transistor 4b, (c) denotes operation of the transistor4c, (d) denotes operation of the transistor 4d, (e) denotes a motorcurrent, (f) denotes an output of the comparator 53, (g) denotes anoutput of the comparator 54, (h) denotes an output of the comparator 57,and (i) denotes a rightward drive inhibit signal ILR and a leftwarddrive inhibit signal ILL of the motor current limiting means 19. Whenthe microcomputer 18 malfunctions and a current starts to flow into themotor, the motor current is increased according to the time constant ofthe motor. When the motor current becomes greater than the predeterminedvalue TH3, the output of comparator 57 is low, the outputs ILR, ILL ofthe motor current limiting means are low, and the transistor 4c is off.When the transistor 4c is off, the motor current is decreased, and whenthe motor current is under the predetermined value TH3, the output ofthe comparator 57 is again high, the rightward drive inhibit ILR andleftward drive inhibit ILL are also high, whereby the transistor 4c ison and the motor current is increased. As the mentioned operations arerepeated, the motor current is limited at the predetermined value TH3.The same operations take place when driving the motor in reversedirection of the steering direction.

As described above, in embodiment 1, since the motor current forproducing the auxiliary force in the reverse direction of the steeringtorque can be applied when the detected motor current value IMD is lessthan the predetermined value TH3, it is possible to improve steeringfeel by the inertia compensation means. On the other hand, since themotor current is limited when the detected motor current value IMD isgreater than the predetermined value TH3, the motor current is limitedto be less than the predetermined value TH3 even when the microcomputer18 is out of order and the motor is driven irrespective of the steeringtorque. The predetermined value TH3 is a current value (3 Å, forexample) large enough to compensate for the inertia of the motor 1, butnot so large current as to bring about a rotation of the steering wheel.Therefore, there is no danger even when motor current flows in adirection which opposes the steering operation. In other words, steeringfeel can be improved without negatively affecting safety.

The same advantage as above can be achieved even when, in thecharacteristic of the motor current limiting means 19, the thresholdvalue TH1 is in the region left of neutral and the threshold value TH2is in the region right of neutral as shown in FIG. 10, or when boththreshold values TH1 and TH2 are neutral as shown in FIG. 11.

Embodiment 2

How an embodiment which is useful at the time of returning the steeringwheel to the neutral is hereinafter described as embodiment 2. Inembodiment 2, the inertia compensation means 101 of embodiment 1 shownin FIG. 6 is replaced with a torque loss compensation means 106, ofwhich a control block diagram is shown in FIG. 12. The remaining partsare the same as embodiment 1 and therefore a further description isomitted herein.

Operation at the time of returning the steering angle to neutral withoutholding the steering wheel after steering right during driving ishereinafter described with reference to FIG. 13. In the drawing, (a)indicates that a state without holding the steering wheel comes at themoment 1 with a steering angle θ of the steering wheel, and that thesteering angle then comes to neutral at the moment 2, (b) indicates asteering torque corresponding to the time shown in (a); (c) indicates atarget value IMT of the motor current corresponding to the steeringtorque TRQ; (d) indicates a current IMC for compensating the torque lossof the electric motor; (e) indicates an indicated value IMI of the motorcurrent obtained by adding the motor current target value IMT and thetorque loss compensation current IMC; and (f) indicates a motor rotationspeed NM.

The torque loss compensation means 106 compensates the torque loss ofthe motor by supplying a current corresponding to the rotation speed NMof the electric motor in the rotational direction of the electric motor.It is certain that the motor current is caused to flow in the reversedirection of the steering torque during the time 3, but since theapplication of current in reverse direction of the steering torque ispossible so long as the motor current is under the predetermined valueTH3, it is possible to cause such a motor current to flow.

As described above, in embodiment 2, since the motor current forproducing the auxiliary force in the reverse direction of the steeringtorque can be applied when the detected motor current value IMD is lessthan the predetermined value TH3, it is possible to improve thecharacteristic of returning the steering wheel by the torque losscompensation means. On the other hand, since no motor current is causedto flow in the reverse direction of the steering torque when thedetected motor current value IMD is greater than the predetermined valueTH3, the motor current is limited to be less than the predeterminedvalue TH3 even when the microcomputer 18, etc. is out of order. Thepredetermined value TH3 is a current value (3 Å, for example) largeenough to compensate the torque loss of the motor 1, but not so large asto bring about a rotation of the steering wheel. Therefore there is nodanger even when motor current flows in a direction which opposes thesteering operation. In other words, steering feel can be improvedwithout negatively affecting safety.

Embodiment 3

Embodiment 3 is hereinafter described with reference to FIG. 14 whichshows a circuit diagram of the control unit of the electric powersteering control system. In FIG. 14, reference numeral 21 denotes ananalog switch for turning on or off an electrical connection between theinput and output of the sample-and-hold circuit 11, the on/off operationbeing controlled by the microcomputer 18. The remaining parts are thesame as FIG. 1 and therefore a further description is omitted herein.

Operation of the microcomputer 18 according to embodiment 3 of theinvention shown in FIG. 14 is hereinafter described with reference tothe flow chart shown in FIG. 15. A steering torque signal TRQ detectedby the torque sensor 15 is applied to the microcomputer 18 in step S1,and then a vehicle speed signal VSP detected by the vehicle speed sensor16 is applied to the microcomputer 18 in next step S2. In step S3, themicrocomputer 18 decides a target value IMT of the motor current inaccordance with the characteristic shown in FIG. 7 based on the steeringtorque signal TRQ and the vehicle speed signal VSP applied thereto, andperforms a PWM control of the motor current so that the value IMDdetected by the motor current detecting value is equal to the targetvalue IMT. In step S4, it is judged whether or not the target value ofthe motor current is zero, and if it is judged zero (YES), the flowadvances to step S6 to turn on the analog switch 21; if the target valueIMT of the motor current is judged other than zero (NO), the flowadvances to step S5 to turn off the analog switch 21. In this manner,detection of the motor current is possible at all times through stepsS4, S5, S6. In step S7, detection of abnormality in the motor current isperformed, and further in step S8, it is judged whether or not such anabnormality is detected. When there is no abnormality, the flow branchesfrom step S8 to step S1 to repeat the processing.

On the other hand, if an abnormality in the motor current is detected,the flow branches from step S8 to step S9 and the microcomputer 18 turnsoff the motor 1, clutch 13 and relay 3 shown in FIG. 14, wherebyprocessing for stopping the power steering control is performed. Theprocessing flow chart of the abnormal motor current in step S7 of FIG.15 is hereinafter described with reference to FIG. 16. In FIG. 16, whichshows the abnormal motor current detecting step S7, it is judged in stepS20 whether or not the absolute value of the difference between thetarget value IMT of the motor current and the detected value IMD of themotor current is greater than a predetermined value (5 Å, for example),and if it is judged YES, it is further judged in step S21 whether or notthe state has continued for a predetermined time (50 ms, for example).If the state is judged continuous (YES), the motor current is judgedabnormal in step S22. If it is judged NO in either of steps S20 and S21,the motor current is considered normal and the flow advances to the nextstep.

In this embodiment, the analog switch 21 is turned on when the motor isoff when PWM signal is low and the analog switch 8 of thesample-and-hold circuit 11 is off, i.e., when the motor 1 is not driven,whereby the section of sample-and-hold circuit 11 is turned on, makingit possible to detect abnormality in the motor current as in step S7.

Embodiment 4

Embodiment 4 of the invention is an example of a safety improvementagainst trouble in or failure of the transistor drive circuits 5a to 5dand the AND circuits 6a, 6b shown in FIG. 1, and is hereinafterdescribed with reference to FIG. 17. This drawing shows a motor currentlimiting means 19, NOT circuits 22, 25 and transistors 23, 24, 26, 27respectively added to FIG. 14 showing the foregoing embodiment 3, and inwhich the transistors 23, 24 are turned on to forcedly turn off thetransistors 4a, 4c when the rightward drive inhibit signal ILR forinhibiting the rightward drive of the electric motor is low. Further,the transistors 26, 27 are turned on to forcedly turn off thetransistors 4b, 4d when the leftward drive inhibit signal ILL forinhibiting the leftward drive of the motor is low. In addition, theprocessing manner is the same as that of the foregoing embodiment 3 andtherefore its description is omitted.

As described above, by employing the circuit of above arrangement shownin FIG. 17, the motor current is set to be less than the predeterminedvalue TH3 even when the motor current flows in the reverse direction ofsteering torque due to trouble in the transistor drive circuits 5a to 5dand the AND circuits 6a, 6b. Furthermore, since the motor current isdetected by turning on the analog switch 21 even when the motor currentis zero and the PWM signal is low, the motor current limiting means 19can be effectively operated even when the motor is off.

Though the inertia compensation control, torque loss compensation, etc.are not mentioned in this embodiment, it is preferable to incorporatethem into this embodiment.

Embodiment 5

Embodiment 5 of the invention is an example of forcedly setting themotor current indicated value IMI to zero when the motor 1 shown in FIG.1 is inhibited from being driven, and is hereinafter described withreference to FIGS. 18 to 20. In addition, the entire arrangement of thecircuit is the same as that of embodiment 1 shown in FIG. 1, FIG. 2 andFIG. 6 and therefore its description is omitted. Operation ishereinafter described with reference to the flow chart shown in FIG. 18.

At the start of this flow chart, a steering torque TRQ detected by thetorque sensor 15 is delivered to the microcomputer 18 in step S1, andthen a vehicle speed VSP detected by the vehicle speed sensor 16 isdelivered to the microcomputer 18 in step S2. In the same manner, themotor speed NM detected by the motor rotation speed sensor 17 is appliedto the microcomputer 18 in step S40. Then the microcomputer 18 performsa required processing based on the data delivered thereto in step S41,and derives an indicated value IMI of the motor current using the targetcurrent deciding means 100 and the inertia compensation means 101.

Then it is judged in step S42 whether or not the indicated value of themotor current derived in this manner is less than a threshold value TH3(3 Å, for example) in the motor current limiting means 19. If it isjudged in step S42 that the indicated value IMI of the motor current isgreater than the predetermined value TH3 (NO), then it is judged in nextstep S43 whether or not the steering torque signal TRQ is greater thanthe predetermined value TH1 set in the motor current limiting means 19(for example, when the steering torque indicates a right torque over1N·m). If it is judged in step S43 that the steering torque signal TRQis greater than the predetermined value TH1 (NO), then it is judged instep S44 whether or not the steering torque signal TRQ is less than thepredetermined value TH2 (for example, when the steering torque indicatesa left torque over 1N·m). If it is judged YES in any of steps S42, S43,or S44, the indicated motor current value is set to the value derived instep S41.

Further, if it is judged that the indicated motor current value isgreater than the threshold value TH3 (NO) in step S42 and that thesteering torque signal TRQ is between the threshold values TH1 and TH2,then the indicated value IMI of the motor current is set to zero in stepS45. Then, the motor current is controlled in step S46 by a PWM signalso that the indicated motor current value IMI set through any of stepsS42 to S45 is equal to the detected motor current value IMD.

In the same manner as in steps S7 to S9 of embodiment 3, the processingof motor current abnormality detection is performed in step S7, and ifthere is no abnormality in the motor current, the flow branches in stepS8 to step 1 to repeat the processing. If an abnormality in the motorcurrent is detected, the flow branches in step S8 to step S9 where themicrocomputer 18 turns off the motor 1, clutch 13 and relay 3, wherebythe electric power steering control is stopped.

In this embodiment, since it is judged in steps S42, S43, S44 whether ornot the motor drive is inhibited and the indicated value IMI of themotor current is limited to zero when the motor drive is inhibited instep S45, it is not necessary to have a processing step in which currenterror is detected by checking for a predetermined value (5 Å, forexample).

Further in this embodiment, though the motor current limiting means 19is based on both the steering torque TRQ and the detected motor currentvalue IMD, it is preferable to also employ the motor current controlmeans based on the steering torque TRQ as shown in FIG. 19 to achievethe same advantage by performing control as shown in the flow chart inFIG. 20.

More specifically, the step S42 in the flow chart in FIG. 18 is omitted,and after performing the motor current target value deciding processingin step S41, if the steering torque signal TRQ is between the thresholdvalues TH1 and TH2 (i.e., when step S44 is NO) as shown in steps S43,S44, the indicated value IMI of the motor current is set to zero in stepS45. Thereafter, in the same manner as the flow chart in FIG. 18, stepsfor controlling the motor current are performed from step S46.

Embodiment 6

Embodiment 6 of the invention monitors the leftward and rightward driveinhibit signals ILL, ILR by the microcomputer 18, and is hereinafterdescribed with reference to FIG. 21. This drawing shows a circuit likethe one shown in FIG. 1, and this circuit is arranged so that therightward drive inhibit signal ILR and the leftward drive inhibit signalILL of the electric motor 1 can be monitored by the microcomputer 18.Buffers 28, 29 are added for adjusting the input signal level of themicrocomputer 18 to the level of the rightward drive inhibit signal ILRand the leftward drive inhibit signal ILL, and the rightward driveinhibit signal ILR is input to an input port PI and the leftward driveinhibit signal ILL is input to an input port P2. The remaining parts arethe same as the foregoing embodiment 1 and a description thereof istherefore omitted.

Operation of this embodiment shown in FIG. 21 is hereinafter describedwith reference to the flow chart in FIG. 22. At the start of this flowchart, a steering torque signal TRQ detected by the torque sensor 15 isinput to the microcomputer 18 in step S1, and then a vehicle speedsignal VSP detected by the vehicle speed sensor 16 is input to themicrocomputer is in step S2. Likewise in step S40, a motor rotationspeed signal NM detected by the motor rotation speed sensor 17 is inputto the microcomputer 18. Then in step S41, the microcomputer 18 performsa processing based on the data delivered thereto, derives an indicatedvalue IMI of the motor current in accordance with the target currentdetecting means 100 and inertia compensation means 101, and in step S46performs PWM control of the motor current so that the indicated valueIMI of the motor current is equal to the detected value IMD of the motorcurrent. Further, in step 550, processing for detecting abnormality ofthe motor current limiting means 19 is performed, and if no abnormalityis detected in step 550, the flow branches in step S51 to step 1 torepeat the processing steps. If an abnormality is detected in step S50,the flow branches in step 51 to step 9 where the microcomputer 18performs processing for turning off the motor 1, clutch 13 and relay 3to stop the electric power steering control.

The abnormality detection processing of the motor current limiting means19 is hereinafter described in detail with reference to the flow chartshown in FIG. 23. First, it is judged in step S60 whether or not theindicated value of the motor current is under the predetermined valueTH3, and if it is judged YES, the abnormality detection processing ofthe motor current limiting means 19 is not necessary and is terminated.If the indicated value IMI of the motor current is greater than thepredetermined value TH3, the flow branches in step S60 to step S61. Thenit is judged in step S61 whether or not the steering torque signal TRQis greater than the predetermined value TH1 as to the rightward drive ofthe motor, and if it is judged YES, then it is further judged in nextsteps S62, S63 whether or not the port P1 remains continuously high fora predetermined time (0.2 sec, for example). If it is judged that theport P1 does remain high for this period, it is judged in step S64 thatthe motor current control means 19, i.e., the rightward drive inhibitmeans, are abnormal. This serves as a first trouble judgement means.Further, with regard to the leftward drive of the electric motor, it isjudged in step S65 whether or not the steering torque signal TRQ isgreater than the predetermined value TH2, and if it is judged YES, it isfurther judged in steps S66, S67 whether or not the port P2 remainscontinuously high for a predetermined time (0.2 sec, for example). Andif it is judged that the port P2 remains high for this period, it isfurther judged in step S68 that the motor current limiting means 19,i.e., the leftward drive inhibit means, are abnormal. This serves as asecond trouble judgment means.

When either the first or second trouble judgment means judges theoccurrence of trouble, it is possible to detect an abnormality of themotor current limiting means 19. The electric power steering control isstopped when the motor current limiting means 19 are abnormal, wherebysafety can be improved.

Embodiment 7

Embodiment 7 of the invention stops the electric power steering controlby detecting an abnormality of the leftward and rightward drive inhibitsignals ILL, ILR and is hereinafter described with reference to FIGS. 24and 25. This drawing is a circuit diagram similar to that of FIG. 1, buthas an AND circuit 30 serving as judgment means and a buffer 31 forconforming the input signal level of the microcomputer 18 to the outputlevel of the AND circuit 30, so that the AND circuit logically multiplesthe rightward drive inhibit signal ILR and leftward drive inhibit signalILL and inputs the resulting signal to an input port P3. The remainingparts are the same as the foregoing embodiment 1 and a descriptionthereof is therefore omitted. The basic processing is the same as thatof embodiment 6 shown in the flow chart in FIG. 22, and is therefore notdescribed.

Abnormality detection processing (which corresponds to step S50 in FIG.22) of the motor current limiting means 19 is hereinafter described withreference to the flow chart of FIG. 25. In the abnormality detectionprocess S50A of the motor current limiting means, first it is judged instep S70 whether or not the indicated value IMI of the motor current isless than the predetermined value TH3, and if it is the abnormalitydetection processing of the motor current limiting means 19 is notnecessary and is terminated. If the indicated value IMI of the motorcurrent is greater than the predetermined value TH3, the flow branchesto step S71 where it is judged whether or not the port P3 is high. Ifport P3 is judged high, it is further judged in step S72 whether or notthe high state remains continuous for a predetermined time (0.2 sec, forexample). And, if the judgement is affirmative, then it is determined instep S73 that the motor current control means 19 is abnormal.

In step S70, when the indicated value IMI of the motor current isgreater than the predetermined value TH3 (3 Å, for example), if thesteering torque signal TRQ indicates right, the rightward drive inhibitsignal ILR is high and the leftward drive inhibit signal ILL is low. Onthe other hand, if the steering torque signal TRQ indicates left, ILR islow and ILL is high. In this manner, when the indicated value IMI of themotor current is greater than the predetermined value TH3, either ILR orILL is low, and the output of the AND circuit 30 is low, so that thesignal applied to the input port P3 of the microcomputer 18 is low.However, when the motor current limiting means 19 is malfunctioning sothat the motor drive inhibit signal in the reverse direction of thesteering torque is high, both ILR and ILL are high and the signalapplied to the input port P3 from AND circuit 30 is also high. Thisindicates an abnormality of the motor current limiting means 19 ineither the rightward drive inhibit means or the leftward drive inhibitmeans. When the motor current limiting means 19 are judged abnormal, theelectric power steering control is stopped, thereby enhancing safety. Inaddition, in this embodiment, there is a further advantage in that onlyone input port is sufficient for monitoring the rightward drive inhibitsignal ILR and leftward drive inhibit signal ILL.

Embodiment 8

In embodiment 8 of the invention the motor and the electric powersteering control are stopped, irrespective of the steering torque, dueto an abnormality of the microcomputer. This embodiment is hereinafterdescribed with reference to FIG. 26. In the drawing, reference numeral32 denotes motor drive limiting means for monitoring the operation ofthe microcomputer 18 and stopping the motor drive due to an abnormality,and numerals 6c, 6d, 33, and 34 denote AND circuits. According to thisarrangement, the electric motor is stopped by the AND circuits 6c, 6dwhen MTOFF signal from the motor drive limiting means 32 is low, and therelay 3 is turned off by the AND circuit 33 and the clutch 13 is turnedoff by the AND circuit 34 when PSOFF signal is low.

The circuit of the motor drive limiting means 32 shown in FIG. 26 ishereinafter described in detail with reference to FIG. 27. In thedrawing, reference numerals 50, 51, and 52 denote resistors by which thepredetermined values TH1, TH2 are decided. Numeral 70 denotes acomparator for comparing the steering torque signal TRQ with thepredetermined value TH1, and the output signal of the comparator 70 isas shown in FIG. 28(b). The output signal TRQ of the torque sensor 15 isthe same as that of embodiment 1 shown in FIG. 3, and accordingly, thepredetermined value TH1 is a threshold value (1N·m of rightwardsteering, for example) of rightward torque, and the predetermined valueTH2 is a threshold value (1N·m of leftward steering, for example) ofleftward torque.

The characteristic of the motor current is shown in FIG. 7, and themotor is driven rightward when the steering torque TRQ is greater thanthe predetermined value TH1, and the motor is driven leftward when thesteering torque TRQ is less than the predetermined value TH2. The outputsignal of the comparator 70 is therefore low when the motor is drivenleftward and high at any other time. Reference numerals 72 and 73 denoteAND circuits, and numeral 74 denotes an OR circuit arranged as shown inFIG. 27. Therefore, the output signal of the OR circuit 74 is higheither if the microcomputer 18 drives the electric motor rightward whenthe torque signal TRQ is less than the predetermined value TH1, or ifthe microcomputer 18 drives the motor leftward when the torque signalTRQ is greater than the predetermined value TH2. In other words, theoutput signal is high when the direction of torque is different fromthat of the motor.

Numeral 75 denotes a resistor and 76 denotes a capacitor, by which theoutput signal of the OR circuit 74 is smoothed. Numerals 77 and 78denote resistors for determining a predetermined value TH4, andcomparator 80 compares the output signal of the OR circuit 74 with asmoothed signal SIG1. When SIG1 is greater than the predetermined valueTH4, the MTOFF signal is low. The MTOFF signal is further input to atimer 81, and when the low state continues for a predetermined time (0.2sec, for example), the PSOFF signal is low until the power source isturned off by a latch circuit 82. The remaining parts are arranged thesame as in the foregoing embodiment 1, and therefore further descriptionis omitted herein.

Operation of this embodiment is hereinafter described assuming the samecondition as in embodiment 1. In FIG. 29, (a) denotes a steering angleθ; (b) denotes an indicated value IMI of the motor current; (c) denotesa PWM signal; (d) is a rightward drive signal R; (e) is a leftward drivesignal L; (f) is an output signal of the OR circuit 74; (g) is asmoothed signal obtained by smoothing (f); (h) is a MTOFF signal; (i) isa PSOFF signal; and (j) is a steering torque signal.

The steering operation is started at time 1 in the same manner asembodiment 1, and when the steering wheel stops at time 2, a motorcurrent flows as shown in (b) to compensate for the inertia of themotor. At time 2, a current flows for a moment in the reverse directionof the steering torque. Accordingly, the output of the OR circuit 74 ishigh for that moment. However, the smoothed signal SIG1 has not reachedthe predetermined value TH4 and, therefore, the MTOFF signal and PSOFFsignal remain high. Thus, as the motor control is not limited,deterioration of steering "feel" caused by inertia is reduced.

Described hereinafter is a malfunction in which the motor iscontinuously driven in one direction irrespective of the steering torquedue to imperfect contact of the torque signal input terminal of themicrocomputer 18, a malfunction of the port for motor drive, or thelike. For example, a rightward drive condition without any steering isshown in FIG. 30. In this drawing, (a) denotes a PWM signal; (b) is arightward drive signal R; (c) is a leftward drive signal L; (d) is anoutput signal of the comparator 70; (e) is an output signal of thecomparator 71; (f) is an output signal of the OR circuit 74; (g) is asmoothed signal SIG1 obtained by smoothing (f); (h) is a MTOFF signal;(i) is a PSOFF signal; and (j) is a motor current.

When an abnormality take places at the time 1, the PWM output port is inthe PWM operation or fixed to be high and the rightward drive signal Ris high, then output of the OR circuit is high, the capacitor 76 ischarged, and the signal SIG1 is thereby increased. When the signal SIG1is greater than the predetermined value TH4, the MTOFF signal is low.Since the MTOFF signal is connected to the AND circuits 6c, 6d, thetransistor 4c is turned off to stop the motor. Further, when such astate continues for a predetermined time T (0.2 sec, for thisembodiment), the latch circuit 82 is latched and the PSOFF signal islow, control of the power steering is stopped until the relay and clutchare turned off to disconnect the power source.

Described hereinafter is a case in which the motor is drivenintermittently irrespective of the steering torque due to imperfectcontact of the microcomputer 18 or due to a malfunction of themicrocomputer 18. For example, FIG. 31 shows a case in which the motoris driven intermittently rightward when there is no steering torque. Inthis drawing, (a) denotes a PWM signal, (b) denotes a rightward drivesignal, (c) denotes a leftward drive signal, (d) denotes the outputsignal of the comparator 70, (e) denotes the output signal of thecomparator 71, (f) denotes the output signal of the OR circuit 74, (g)denotes the signal SIG1 obtained by smoothing signal (f), (h) denotesthe MTOFF signal, (i) denotes the PSOFF signal, and (h) denotes a powersource.

A rightward drive is repeated during the periods indicated by "I" andthe motor is off during the periods indicated by "II" as a result of amalfunction, and therefore the output of the OR circuit 74 cycles highand low. When the smoothed signal SIG1 is gradually raised and exceedsthe predetermined value TH4, the MTOFF signal is low and the transistor4c is forcedly turned off, thereby stopping the motor 1. If such atrouble state continues for a predetermined time (0.2 sec, for example),the latch circuit 82 is latched low, the PSOFF signal is low, and therelay 3 and the clutch 13 are turned off, whereby the control of thepower steering is stopped until the power source is turned off.

As described above, in embodiment eight, if the motor is driven in thereverse direction of the steering torque during the normal state, thereverse drive is not inhibited and, therefore, it becomes possible toimprove the steering feel by inertia compensation or the like. On theother hand, if the electric motor is driven irrespective of the steeringtorque due to a malfunction of the microcomputer, the motor is stoppedand, furthermore, control of the power steering is stopped. As a result,safety is improved. Further, even if the reverse direction drive isgenerated intermittently, since an abnormal output is judged bycomparing the level of the smoothed signal SIG1 which is the OR circuitoutput signal showing a generation of the reverse direction drive withthe predetermined value TH4, when the duration of the reverse directiondrive generation time becomes large when compared to the time duringwhich the reverse direction drive is not generated (time T2), the levelof the smoothed signal SIG1 exceeds the predetermined value TH4, wherebyabnormality can be detected. On the contrary, when the duration of thereverse direction non-generation time T1 becomes small when compared tothe reverse direction non-generation time T2, the level of the smoothedsignal SIG1 does not exceed the predetermined value TH4 wherebyabnormality cannot be detected. In this case, the motor current flowsintermittently in the reverse direction against the steering torque, butan excessively large torque is not produced because the predeterminedvalue TH4 is set such that the average value of the intermittent motorcurrent in such a case will be small (3 Å, for example, in thisembodiment), whereby safety is not affected. This arrangement iseffective even when the motor current detecting means is out of orderand the detected motor current value is zero at all times.

Embodiment 9

Embodiment 9 of the invention provides a threshold value of the detectedmotor current value IMD with a hysteresis, and is hereinafter describedwith reference to FIGS. 32-34. This embodiment is a modification of themotor current limiting means 19 of embodiment 1 as shown in FIG. 32. Thecircuit in FIG. 32 is arranged by adding a resistor 83 to the embodimentof FIG. 2 and the threshold value of the motor current is provided witha hysteresis. The input/output characteristic is as shown in FIG. 33.Accordingly, in the motor current limiting means 19, when the detectedmotor current value IMD is less than a predetermined value TH5 (3 Å, forexample), both rightward drive inhibit signal ILR and leftward driveinhibit signal ILL are high, and when the detected motor current valueIMD is greater than the predetermined value TH5, the rightward driveinhibit signal ILR and leftward drive inhibit signal ILL havecharacteristics as shown in FIG. 4. When the detected motor currentvalue IMD is less than a predetermined value TH6 (2 Å, for example),both rightward and leftward drive inhibit signals ILR, ILL are againhigh.

Operation of this embodiment is hereinafter described with reference toFIG. 34. In this drawing, (a) denotes operation of the transistor 4a;(b) denotes operation of the transistor 4b; (c) denotes operation of thetransistor 4c; (d) denotes operation of the transistor 4d; and (e)denotes an indicated motor current value IMD. FIG. 34 shows a state inwhich a trouble occurs in the microcomputer 18 at the time 1 when thesteering torque is neutral and the motor is in the rightward drivecondition. When falling in the rightward motor drive condition, themotor current is increased at an inclination according to the timeconstant of the electric motor. Since the steering torque is neutral,the rightward and leftward drive inhibit signals ILR, ILL are both low,and the transistor 4c is turned off at the time 2, whereby the detectedmotor current value IMD is reduced forming an inclination according tothe time constant of the motor. When the detected motor current valueIMD is less than the predetermined value TH6, the output of thecomparator 57 is again high, and both rightward and leftward driveinhibit signals ILR, ILL are high, whereby the transistor 4c is turnedon and the motor current is increased again from the time 3 to repeatthe same operation thereafter.

As described above, by providing the threshold value of the detectedmotor current value with a hysteresis, the frequency with which thetransistor cycles on and off can be lowered. If the on/off cyclingfrequency of a transistor is high, the transistor cannot follow andoperates within its active region, possibly resulting in damage to thetransistor. In embodiment 9, however, this problem can be avoided byrestricting the on/off frequency of the transistor. This frequency canbe restricted to be less than 30 kHz, for example. In addition, the sameadvantages as exhibited by embodiment 1 are also achieved.

As has been described above, the invention has the following effects.

(1) Since it is possible to drive in the reverse direction of thesteering torque while limiting the motor current, steering feel can beimproved, and motor control can be performed without affecting safety.

(2) Since it is possible to drive in the reverse direction of thesteering torque, improvement in steering feel is easily performed bymeans of inertia compensation control, torque loss compensation control,etc. Since the current applied in the reverse direction of the steeringtorque is a small current which serves only to compensate the inertia ortorque loss of the motor, a dangerous torque is not produced when such acurrent is applied in the reverse direction of the driving torque. As aresult, steering feel can be improved without affecting safety. The sameadvantage can also be achieved with regard to phase compensation.

(3) An electric power steering control system is disclosed in whichmotor current is detected by a motor current detecting circuit using asample-and-hold circuit and an abnormality in the motor current isdetected in accordance with a detected motor current value, and sincethe detection of motor current is possible when the motor is not drivenby turning on the sample-and-hold circuit, detection of an abnormalityin the motor current becomes possible, resulting in a safer electricpower steering control system.

(4) An electric power steering control system is disclosed comprisingmotor current detecting means for detecting a motor current using asample-and-hold circuit, abnormal motor current detecting means fordetecting abnormality in the motor current in accordance with thedetected motor current value, and motor current limiting means forlimiting the motor current in accordance with the steering torque anddetected motor current value. Since detection of motor current ispossible when the motor is not driven by turning on the sample-and-holdcircuit, detection of an abnormality in the motor current is possible.Furthermore, it is possible to limit the motor current in the reversedirection of the steering torque by the motor current limiting means,resulting in a safer electric power steering control system.

(5) Because the target value of the motor current is established to bewithin a range limited by the motor current limiting means a currenterror does not take place between the indicated value of motor currentand the detected value of motor current. Therefore erroneous detectionof an abnormality in the motor current is eliminated, resulting in amore reliable electric power steering control system.

(6) Any abnormality in the motor current limiting means is detected, andif an abnormal condition is detected therefrom, the control of electricpower steering is stopped, resulting in a safer electric power steeringcontrol system.

(7) There is a further advantage that because only one signal isemployed for detecting abnormality in the motor current limiting means,only one input port of the microcomputer is required.

(8) Since the motor is stopped when a percentage of time during whichthe motor is driven in a reverse direction of the steering torque isgreater than a predetermined value, reverse drive is possible as long asthe percentage of reverse drive time is small, as for example, in thecase of inertia compensation, steering feel is improved. In case themotor is continuously or intermittently driven irrespective of steeringtorque due to a malfunction in the microcomputer, the percentage ofreverse drive time is increased and driving of the motor is stopped,thereby advancing safety. In case the motor is driven irrespective ofsteering torque intermittently for a small percentage of time, the motordrive is not stopped. However, the current passing through the electricmotor in such a case is so small that a dangerous situation does notdevelop. The control system is operated effectively even when the motorcurrent detecting means is out of order and the detected motor currentvalue is zero at all times. In other words, steering feel can beimproved without affecting safety.

(9) At the time of limiting the motor current, the driving frequency ofthe transistor which drives the motor is controlled so as to be lessthan a predetermined value, whereby operation of the transistor withinthe active region is avoided, thus preventing damage to the transistor.

The invention has been described with respect to certain preferredembodiments. Various modifications and additions within the spirit ofthe invention will occur to those of skill in the art.

What is claimed is:
 1. An electric power steering control systemcomprising:steering force detecting means for detecting a steeringforce; an electric motor for producing an auxiliary steering force;motor current detecting means for detecting a motor current of saidelectric motor; motor current control means for determining a motorcurrent value and a driving direction of the electric motor required bysaid electric motor at least in accordance with the steering forcedetected by said steering force detecting means, thereby controlling themotor current; rightward motor drive means for driving said electricmotor rightwardly; leftward motor drive means for driving said electricmotor leftwardly; rightward drive inhibit means having a function ofoutputting a rightward drive inhibition signal ILR and forcedly turningoff said rightward motor drive means when said steering force is in arightward motor drive inhibit region; leftward drive inhibit meanshaving a function of outputting a leftward drive inhibition signal ILLand forcedly turning of f said leftward motor drive means when saidsteering force is in a leftward motor drive inhibit region; motor drivepermit means for stopping the inhibit function of said rightward driveinhibit means and said leftward drive inhibit means and permitting theelectric motor to drive when a detected value of said motor current isless than a predetermined value; first trouble judgment means fordetecting said leftward drive inhibit signal when said motor current isgreater than the predetermined value and said steering force isrightward, and, if the leftward drive inhibit signal is detected whensaid motor current is greater than the predetermined value and saidsteering force is rightward, judging that the leftward drive inhibitmeans is abnormal when leftward drive is permitted; and second troublejudgment means for detecting said rightward drive inhibit signal whensaid motor current is over the predetermined value and said steeringforce is leftward, and, if the rightward drive inhibit signal isdetected when said motor current is greater than the predetermined valueand said steering force is leftward, judging that the rightward driveinhibit means is abnormal when rightward drive is permitted; whereinsaid motor control is stopped when one of said first trouble judgmentmeans and said second trouble judgment means judges an abnormalcondition.
 2. An electric power steering control systemcomprising:steering force detecting means for detecting a steeringforce; an electric motor for producing an auxiliary steering force;motor current detecting means for detecting a motor current of saidelectric motor; motor current control means for determining a motorcurrent value and a driving direction of the electric motor required bysaid electric motor at least in accordance with the steering forcedetected by said steering force detecting means, thereby controlling themotor current; rightward motor drive means for driving said electricmotor rightwardly; leftward motor drive means for driving said electricmotor leftwardly; rightward drive inhibit means having a function ofoutputting a rightward drive inhibition signal ILR and forcedly turningoff said rightward motor drive means when said steering force is in arightward motor drive inhibit region; leftward drive inhibit meanshaving a function of outputting a leftward drive inhibition signal ILLand forcedly turning off said leftward motor drive means when saidsteering force is in a leftward motor drive inhibit region; motor drivepermit means for stopping the inhibit function of said rightward driveinhibit means and said leftward drive inhibit means and permitting theelectric motor to drive when a detected value of said motor current isunder a certain value; and judgment means for judging whether or notboth said rightward drive inhibition signal and said leftward driveinhibition signal are permitted; wherein one of the rightward driveinhibit means and the leftward drive inhibit means judges occurrence ofan abnormal condition to stop said motor control when said motor currentis greater than a predetermined value and said judgment means judge thatboth signals are permitted.
 3. An electric power steering control systemcomprising:steering force detecting means for detecting a steeringforce; an electric motor for producing an auxiliary steering force;motor current control means for determining a motor current value and adriving direction of the electric motor required by said electric motorat least in accordance with the steering force detected by said steeringforce detecting means, thereby control ling the motor current; rightwardsteering detecting means for detecting rightward steering in accordancewith said steering force; leftward steering detecting means fordetecting leftward steering in accordance with said steering force; andmotor drive limiting means for detecting a first state in which saidrightward steering detecting means detect the rightward steering andsaid motor control means drive said electric motor leftwardly, and asecond state in which said leftward steering detecting means detect theleftward steering and said motor control means drive said electric motorrightwardly, and stopping the control of said electric motor when apercentage of time during which one of said first and second states isdetected is greater than a predetermined value.
 4. An electric powersteering control system according to claim 3, further comprising motorcurrent limiting means for limiting the motor current to be less than apredetermined value by turning off the electric motor when the detectedmotor current value is greater than a predetermined value, and in whichat the time of limiting the motor current, a driving frequency of atransistor for driving said electric motor is controlled so as to beless than a certain value.